A receiver, also known as a User Equipment (UE), mobile station, wireless terminal and/or mobile terminal, is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
Wireless communication systems are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc. These wireless systems may be multiple-access networks capable of supporting multiple users by sharing the available network resources. Examples of such multiple-access systems may comprise, e.g., Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA) networks, just to mention a few arbitrary examples. It may be noted that the terms “network” and “system” are often used interchangeably within the present context.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The receivers in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. Sometimes, also the expression cell may be used for denoting the radio network node itself. However, the cell is also, or in normal terminology, the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the receivers within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the receiver to the radio network node.
The downlink of the 3GPP Long Term Evolution (LTE) cellular communication system is based on Orthogonal Frequency Division Multiplex (OFDM) transmission, which uses time and frequency resource units for transmission. The smallest time-frequency resource unit, called Resource Element (RE), comprises a single complex sinusoid frequency (sub-carrier) in an OFDM symbol. For the purpose of scheduling transmissions to the different receivers, the resource elements are grouped into larger units called Physical Resource Blocks (PRBs).
A Resource Element (RE) may convey a complex-valued modulation symbol on a subcarrier. In this context, the RE may be referred to as time-frequency resources, or time-frequency resource elements. A Resource Block (RB) comprises a set of resource elements or a set of time-frequency resources and is of 0.5 ms duration (e.g., 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). The LTE standard refers to a PRB as a resource block where the set of OFDM symbols in the time-domain and the set of subcarriers in the frequency domain are contiguous. The LTE standard further defines Virtual Resource Blocks (VRBs) which can be of either localized or distributed type. For brevity, sometimes only the notion of resource block is used and a skilled reader would be able to determine the proper term. The transmission bandwidth of the system is divided into a set of resource blocks. Typical LTE carrier bandwidths correspond to 6, 15, 25, 50, 75 and 100 resource blocks.
A PRB occupies a half of a subframe, called “slot”, comprises six or seven consecutive OFDM symbol intervals in time domain (0.5 millisecond in total), and twelve consecutive sub-carrier frequencies in frequency domain (180 kHz in total). Each PRB is indicated by a unique index nPRBε[0, NRBDL−1] denoting the position of the sub-band that the PRB occupies within a given bandwidth, where NRBDL is the total number of PRB within the bandwidth. The maximum number of PRBs NRBmax,DL, associated with the largest LTE bandwidth (20 MHz), is 110. The relation between the PRB number nPRB in the frequency domain and resource elements (k, l) in a slot is nPRB=└k/NscRB┘.
The LTE Rel-8/10 defines a Physical Downlink Control Channel (PDCCH) as a signal containing information needed to receive and demodulate the information transmitted from the serving cell (i.e., eNodeB in LTE terminology) to a receiver through the Physical Downlink Shared Channel (PDSCH). The PDCCH is transmitted in a control region that can occupy up to four OFDM symbols at the beginning of each subframe, whereas the remaining of the subframe forms the data region used for the transmission of the PDSCH channel.
Each transmission of user data on the Physical Downlink Shared Channel (PDSCH) is performed over 1 ms duration, which is also referred to as a subframe, on one or several resource blocks. A radio frame consists of 10 subframes, or alternatively 20 slots of 0.5 ms length (enumerated from 0 to 19).
OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM is a Frequency-Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier.
The LTE Rel-11 supports a new control channel scheduled within the time-frequency resources of the downlink data region. Unlike the legacy PDCCH, this new feature, known as Enhanced Physical Downlink Control Channel (EPDCCH), has the distinct characteristic of using Demodulation Reference Signals (DM-RS) for demodulation.
The EPDCCH transmission may be either localised or distributed with the granularity of one PRB pair. With localised transmission, the EPDCCH for a receiver is preferably transmitted over a single PRB pair, or, in some cases, over a few consecutive PRB pairs, scheduled by the associated eNodeB based on Channel Quality Indicator (CQI) feedback information (frequency selective scheduling). CQI is a measurement of the communication quality of wireless channels. CQI may comprise a value, or values, representing a measure of channel quality for a given channel. Typically, a high value CQI is indicative of a channel with high quality and vice versa.
With distributed transmission, the EPDCCH is transmitted over multiple PRB pairs spread over the downlink system bandwidth to achieve frequency diversity. The latter scheme is useful if there is no feedback or the available feedback is not reliable, although more resources (i.e., PRBs) are locked for EPDCCH transmission.
The EPDCCH design is based on a mobile station specifically configured search space. In particular, for a given receiver, the serving network node (i.e., eNodeB) may allocate up to two sets of PRB pairs for EPDCCH transmission (EPDCCH sets in LTE terminology), where each set may comprise M={2, 4 or 8} PRB pairs. Each EPDCCH set may be configured for either localized or distributed EPDCCH transmission. To assure that all PRB pairs within an EPDCCH set have the same number of available resource elements for EPDCCH transmission, PRB pairs containing the Physical Broadcast Channel (PBCH) and/or synchronisation signals (PSS/SSS) or positioning reference signals are not utilised for EPDCCH transmission. All available resource elements of each PRB pair within an EPDCCH set are mapped sequentially (first in frequency, then in time) into sixteen Enhanced Resource Element Groups (EREGs), hence yielding EREGs that may differ in size by at least one resource element. The basic unit block for EPDCCH multiplexing and blind decoding, the Enhanced Control Channel Element (ECCE), is formed by grouping a number of EREGs, which are selected either within a single PRB pair (for localised EPDCCH transmission) or across multiple PRB pairs (for distributed EPDCCH transmission) in such a way to assure that all ECCEs within an EPDCCH set have roughly the same size. An EPDCCH is then transmitted using an aggregation of one or several consecutive ECCEs, where the number of ECCEs used for one EPDCCH depends on the EPDCCH format as given by Table 1.
TABLE 1Number of ECCEs for one EPDCCH, NEPDCCHECCECase 1Case 2EPDCCHLocalizedDistributedLocalizedDistributedformattransmissiontransmissiontransmissiontransmission02211144222884431616884—32—16
Future releases of the LTE system will introduce a New Carrier Type (NCT) that may be either synchronised or unsynchronised with the existing legacy carrier. A more ambitious vision is a standalone NCT. A major characteristic of this new carrier is the absence of a legacy downlink control channel region, that is, downlink control information is conveyed primarily through EPDCCH. Therefore, to ensure that a sufficient number of resources are available for EPDCCH, it may be desirable to make all PRB pairs in the downlink frequency bandwidth of the new carrier available to be configured for EPDCCH transmission, despite the presence of signals, such as e.g., reference signals, that may occupy only part of the downlink frequency bandwidth in a given subframe. The consequence is thus that an EPDCCH set may comprise PRB pairs having different number of resource elements available for the EPDCCH. This implies that the sizes of the ECCEs for localised EPDCCH are different among PRB pairs, and are possibly also different for the case of distributed EPDCCH transmission. For instance, for the design of at least an unsynchronised new carrier type, the six central PRB pairs contain PSS/SSS and if a standalone NCT will be specified, these PRB pairs would also be used for the transmission of broadcast information. To avoid waste of resources for small system bandwidths, an EPDCCH set may comprise all or some of the 6 central PRB pairs together with other PRB pairs.
In the prior art LTE system, for the transmission of an EPDCCH to a receiver, a serving network node allocates up to two sets of PRB pairs (i.e., EPDCCH sets in LTE terminology). The available resource elements in each PRB pairs of a set are mapped into 16 Enhanced Resource Element Groups (EREGs), whose size depends on reference signal configurations, such as e.g., Common Reference Signal (CRS), Channel State Information Reference Signals (CSI-RS), Demodulation Reference Signal (DM-RS), subframe type as well as control region length.
CSI-RS is a sparse receiver-specific reference signal used primarily for estimating Channel State Information (CSI) such as, e.g., Channel Quality Indicator (CQI), Pre-coding Matrix Indicator (PMI), Rank Indicator (RI), which the receiver reports to the transmitter/eNodeB. The CSI-RS is transmitted in all resource blocks of the carrier but with a configurable period in time and it is much sparser than the CRS. Up to 8 CSI-RS antenna ports may be accommodated.
Yet another downlink reference signal defined in LTE is Demodulation Reference Signal (DM-RS). DM-RS is a receiver-specific reference signal used primarily as phase and amplitude reference for coherent demodulation, i.e., to be used in channel estimation. It is only transmitted in the resource blocks and subframes where the receiver has been scheduled data, i.e., containing the PDSCH, or downlink control channel, i.e., containing the EPDCCH. Up to 8 DM-RS antenna ports may be accommodated.
The DM-RS time-frequency patterns for LTE are defined in the Technical Specification: 3GPP TS36.211 (retrievable over the Internet from: http://www.3gpp.org).
EREGs are grouped into Enhanced Control Channel Element (ECCE) in a way that different ECCEs have roughly the same amount of resource elements. For decoding the downlink control channel, a receiver blindly decodes the downlink control channel on a set of possible time-frequency resource positions (i.e., EPDCCH candidates in LTE terminology) formed by the aggregation of one or several ECCEs according to Table 1. The number of EPDCCH candidates per aggregation level is specified in the standard and depends on the number and sizes of the EPDCCH sets.
The supported numbers of ECCEs forming and EPDCCH candidate (i.e., the aggregation level) depend on the number of available resource elements in the PRB pairs as given in Table 1, which defines two sets of possible aggregation levels for both distributed and localized EPDCCH transmission. As the current LTE design assures that all PRB pairs within an EPDCCH set have the same number nEPDCCH of resource elements available for EPDCCH transmission, the quantity nEPDCCH characterises the entire EPDCCH set and is used to discriminate between the two sets of aggregation levels. In particular, Case 1 provides a larger value of both the minimum and the maximum aggregation levels for both distributed and localised transmission and applies when:
DCI formats 2, 2A, 2B, 2C or 2D is used and the downlink system bandwidth is larger than 25 resource blocks; or
Any DCI format when nEPDCCH<104 and normal cyclic prefix is used in normal subframes or special subframes with configuration 3, 4, 8.
The threshold value of 104 downlink resource elements available for EPDCCH transmission in a physical resource block pair has been determined to guarantee a worst case code rate of roughly 0.8 when an EPDCCH consists of one ECCE.
To assure that the PRB pairs within an EPDCCH set have the same number of resource elements for EPDCCH transmission, PRB pairs carrying the Physical Broadcast Channel (PBCH), and/or synchronisation signals, such as i.e., PSS/SSS in the LTE system, or positioning signals are not used an EPDCCH set in the LTE Rel.-11.
In future evolution of the LTE system, this design restriction may be relaxed to avoid waste of resources. For instance, for the design of an unsynchronised New Carrier Type (NCT), or a standalone NCT, an EPDCCH set may need to include all or some of the PRB pairs used for the transmission of synchronisation signals and/or broadcast channel to avoid subframes where no control information neither data are transmitted. It could also be envisaged to transmit other signals (e.g., reference signals) over only part of the bandwidth. The consequence is that a serving cell may configure an EPDCCH set formed by PRB pairs having different number of resource elements available for the transmission of the downlink control channel. Thus, both the serving cell transmitting an EPDCCH and the intended receiver must determine unambiguously the aggregation levels that apply to all PRB pairs within an EPDCCH set.